This invention is drawn to the field of microwave optics, and more particularly, to a multi-beam, multi-lens microwave antenna providing hemispheric coverage.
Many Naval applications in electronic warfare and wide-angle surveillance call for a microwave antenna the response pattern of which displays a 360.degree. azimuth and at least a 90.degree. elevation. U.S. Pat. No. 3,755,815, issued Aug. 28, 1973 to Stangel et al, the patentees of which are the present applicants, incorporated herein by reference, provides such an antenna system comprising a planar phased array fed dome lens. Hemispheric coverage is provided by controllably varying the phase of the planar feed array such that the radiation produced by the array is sequentially directed to preselected regions of the dome lens. The action of the dome lens is to refract the radiation producing collimated beams of electromagnetic energy over 360.degree. of azimuth and at least 90.degree. of elevation. The active planar phased array feed technique, however, requires complex and expensive electronic signal processing and microwave coupling modules which suitably phase the planar feed array in the transmit mode for providing the hemispheric beam scanning capability and which, in the receive mode, recover the phase information for identifying the bearing of potential threats.
The Luneberg lens comprises a sphere the index of refraction (.eta.) of which varies as a function of the radial distance from the center of the sphere according to the relation .eta.(r)=(2-r/R).sup.1/2, where R is the radius of the sphere and r is the radial coorindate of any point within the sphere. Such a lens is capable of hemispheric coverage because of the property that a feed source placed adjacent any surface point produces a collimated wavefront on the other side of the sphere travelling in the direction of the line from the feed point through the center of the sphere. However, not only is a sphere having a radially variable index of refraction difficult and expensive to construct but also considerable mechanical difficulties are encountered in controllably scanning the feed source about the spherical surface to provide hemispheric coverage. An array of feed sources positioned around the lower hemisphere up to the equatorial plane produces severe aperture blockage and pattern degradation especially for the low elevation angle beams.